A Study on Structure and Spin Equilibria of Metal Complexes in Solution.

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Bivalent transition metal ions form a series of mononuclear complexes with halide ions in aprotic donor solvents. These metal ions are usually six-coordinated with solvent molecules and the coordination structure around a metal ion changes to four-coordination upon complexation with halide ions. It is suggested that a halogeno complex consists of two equilibrium species with different numbers of coordinating solvent molecules. The equilibrium may change with temperature. Thus, in this work, we investigated temperature dependence of themodynamic parameters of complexation and electronic spectra of individual complexes by using a precise titration calorimetry system and spectrophotometric titration system developed in our laboratory.In dimethylacetamide (DMA), the reaction enthalpy and entropy values of chloro complexation of the nickel (II) ion show remarkable temperature dependence. The result is consistent with that from electronic spectra, suggesting that the monochloro complex consists of [Nicl(DMA)_5]^+ and [NiCl(DMA)_4]^+, and the dichloro complex [NiCl_2(DMA)_3] and [NiCl_2(DMA)_2]. The reaction enthalpy and entropy values of halogeno complexation of the zinc (II) ion suggested that the number of coordinating solvent molecules in the metal solvate ion is less than six, implying that [Zn(DMA)_6]^<2+> and [Zn(DMA)_4]^<2+> are present in DMA. In fact, an EXAFS spectroscopy shows the average coordination number of 4.6.The adduct formation reaction between [Ni(dtp)_2] (dtp = dithiophosphate (C_2H_5)O_2PS_2) with various nitrogen-donor ligands have been thermodynamically and spectrophotometrically studied, because its electronic state changes from low spin to high spin upon complexation.